One of the greatest and most commonly used inventions in the world probably came about because of a very early necessity, possibly even before humans settled down and started to build permanent dwellings. I am speculating because nobody knows, but I imagine that in the days when we were all hunters and gatherers, someone needed to get up a very steep hill. By carving out a few footholds, this task was made considerably easier, and so at some point in our history were made the first set of stairs.

Stairs in Pompeii.

Stairs allow us to safely ascend or descend slopes that would utterly defeat us otherwise unless we had ropes and climbing skill. And we can carry things up and down stairs without needing our arms or hands free. (Unlike ladders, which require holding on with your hands as you climb.) Stairs can lead us to dizzying heights atop our tallest towers with surprising rapidity, as long as your leg muscles hold out. Going up or down less than five floors in my office building, the stairs are almost always faster than the lifts. Just a handful of steps can take you up or down an entire floor within a building. Think how difficult even this small vertical distance would be to cover without stairs.

Stairs up to Sacre Coeur, Paris.

The basic elements of stairs are small horizontal surfaces, large enough to place your feet on, separated by a vertical rise which is a comfortable distance to cover in a single stride. The horizontal surfaces are called treads and the vertical sections (if present) are called risers. A paired tread and riser is a step. A set of steps is a synonym for a set of stairs. The result produces an inclined slope, at an angle roughly 45° to the horizontal, though this angle can vary quite a bit. Some buildings, particularly older ones, have very steep stairs, squeezing a lot of vertical rise into a small horizontal space. This both saves space in cramped buildings and allows building taller structures with less effort and cost. Temples or cathedrals or similar such structures which are meant to look impressive often sacrificed ease of climbing for the sake of structural height. The drawback is that steep steps are difficult to negotiate and can trigger a fear of stumbling and falling.

Spiral stairs, Rathaus tower, Rothenburg ob der Tauber.

To save space even more, stairs can be wound around on top of themselves, creating a spiral staircase. One circuit of the spiral needs to gain enough height to prevent you hitting your head on the bottom of the steps above, so very tight spirals tend to be very steep. Often such spirals can be found in old church towers, assembled out of heavy stone, where the weight seems to press upon your head, defying the fact that the tower has stood for centuries without collapsing. Modern buildings tend to use squared off spirals to fit into our angular architectural spaces, generating the deep stairwells which allow you to peer up and down dozens of storeys and provide dramatic film and television locations.

Sydney Opera House stairs.

Shallow steps, on the other hand, spread out height gain over a greater horizontal distance and can be easier to climb. Normal steps at close to a 45° angle require adopting a stair-climbing stride, which feels fundamentally different to simple walking. The ancient Romans built shallow steps leading up to their temples and fora - the height and width of each step designed so that you could walk up them without breaking from a natural walking stride. When Jørn Utzon designed the Sydney Opera House, he used the same dimensions for the broad steps leading up to this modern temple of arts. Walking up or down those steps is a sensory and tactile pleasure that could not have been equalled with steeper, more conventionally sloped stairs.

National Gallery steps - a meeting place.

Steps can form natural gathering places or tourist attractions. Steps provide a convenient place for people to sit while they wait, or simply people watch. In Sydney one of the most popular meeting places in the CBD[1] is the Town Hall steps. Every evening you can see dozens of people seated or standing on these steps, waiting for their friends. The Opera House steps are not so much a meeting place as a gathering place and a prime spot for tourist photos. The Spanish Steps in Rome, the steps of the National Gallery in London, and the steps winding up and down the Great Wall of China all play a similar role.

Spanish Steps, Rome.

There are many steps leading up to the ancient Incan mountain peak city of Machu Picchu in Peru. There is the magnificent double spiral staircase in the Vatican Museums. There are the Potemkin Stairs in Odessa. Stairs lead the way to the tops of lighthouses and lookout spots the world over. Steps riddle steep seaside villages across Europe, and enable walking trails up and over the Alps and other mountain ranges across the world.

Steps of Machu Picchu, Peru.

We are largely a horizontal species, living our lives on more or less flat land, but enough of our world is vertical that we require a way to deal with altitude changes. Lifts and escalators provide vertical access without requiring muscle power, but stairs had them beat by thousands of years and remain much more common. Next time you climb a set of stairs, marvel at how such a simple invention can lead to so much, both literally and figuratively.

[1] Central business district, the Australian term for "downtown" in the major cities.